1. Field of the Invention
This invention relates to sheave assemblies having multiple pulleys or rollers used for such activities as lowering wire cables to which sensors and instruments are attached, as in hydrographic work from ships where a freely suspended sheave assembly is required.
2. Description of Prior Art
For the above described purpose, the prior art discloses a counter-balanced sheave assembly for supporting a cable having a tension applied thereto comprising a plurality of pulleys (rollers) rotatably mounted along a downwardly facing, arcuate path with each roller having its axis of rotation being substantially perpendicular to a plane containing the arcuate path. Consequently, the force of the cable extending over each roller is directed downward in a direction substantially perpendicular to the pivotal axis of the roller as shown in the prior art sheave assembly of FIG. 1. Examples of such multiple-pulley sheave assemblies are shown in U.S. Pat. Nos. 4,492,363 to Niskin; 4,417,718 to Niskin; and pending U.S. application Ser. No. 824,243 to Niskin et al. now U.S. Pat. No. 4,690,380.
Referring to FIG. 1, shown therein is one of a plurality of prior art rollers 10 having a cable 12. The roller 10 is mounted in a support frame 14 having extension portions 16 extending above the roller on each side of the cable to feed the cable 12 into a groove 18 of the roller 10 and to maintain the cable within the groove. In this manner, the extension portions 16 act as a funnel.
There are situations wherein the prior art sheaves have certain drawbacks. First, for example, there are streamer cables which have portions of varying diameters composed of different materials. These streamers include a small diameter steel cable portion, a PVC plastic cable portion of larger diameter filled with oil, and yet a larger diameter steel housing or module cable portion that can be used for coupling or for instruments. For example, this steel housing could have a four inch diameter. Applicant was faced with two problems when designing a sheave assembly for this type of cable. First, applicant had to design a sheave of reasonable size to accomodate a large diameter cable and also a sheave that could accomodate a cable having portions with substantially varying diameters.
The applicant considered enlarging the rollers and extension portion 16 of FIG. 1 to correspondingly receive the largest cable diameter. First, as a minimum, the groove of the roller must extend up to the equator of the cable and the roller must have a sufficient diameter at the center of the groove to provide structural strength. Consequenty, for a four inch diameter sheave assembly, the groove would have to be a minimum of 2.25 inches and the roller would have to have at least a 5.5 inch diameter. Since the cable must not have more than a three degree bend around each roller, the sheave assembly requires a large number of rollers. Consequently, the use of rollers with a 5.5 inch diameter would create an enormous sheave assembly, which would have a diameter, for example, extended from an initial 9 feet to 15 feet. This size diameter is highly undesirable in its intended environment.
In the above design, there is a second drawback. With the groove of the roller only extending to the equator of the cable, the depth of the groove in the roller is reduced to allow for smaller rollers. Consequently, the extension portions 16 must take the place of reduced walls of the groove. This leads to a second problem of the cable rubbing against the extension portions 16 which are fixed and do not rotate with the rollers. If this problem is eliminated by extending the walls of the groove upward as in the prior art (shown in FIG. 1), then this makes the size of the rollers and therefore the sheave assembly even larger, e.g. 9 feet diameter increased to something in excess of 20 feet diameter.
Additionally, if instead of extending the grooves upward, the extension portions were spaced apart by a greater distance, then the cable would not be reliably fed into the groove of the roller, particularly for a small wire cable.
Consequently, applicant was faced with what appeared to be an insolvable problem, how to have an acceptable size sheave assembly using prior art designs. Additionally, there was a second problem of accomodating cables of substantially varying diameters.